1. Technical Field of the Invention
The present invention relates generally to flow control devices and, more particularly, to a linear displacement control valve including a valve trim outfitted with a uniquely configured spindle which is adapted to provide low end flow control at a higher level of accuracy and/or wider rangeability.
2. Description of the Related Art
In the prior art, one currently know control valve includes a plug or spindle that is linearly displaced during normal operation of the valve. Within these valves, which are often referred to as linear displacement valves, the plug is disposed and moveable within a disc stack or valve cage which defines a multiplicity of tortuous and/or non-tortuous fluid passageways. The valve trim of these valves comprises the combination of the plug and the valve cage. Certain linear displacement valves are configured for “over plug flow” wherein fluid flows radially inward into the interior of the valve cage from the exterior thereof, with the fluid undergoing a pressure drop as a result of the flow through the passageways of the valve cage. In this arrangement, the valve is opened by lifting the plug off a seat ring which thus allows the fluid to flow from the interior of the valve cage and out of the valve via the unblocked seat ring. Conversely, movement of the seating surface of the plug into sealed engagement with the complimentary seating surface of the seat ring facilitates a closed or shut-off condition for the valve.
As an alternative to over plug flow, other linear displacement valves are configured for “under plug flow” wherein fluid flows axially upwardly into the interior of the valve cage to the exterior thereof, with the fluid undergoing a pressure drop as a result of the flow of the fluid radially outwardly through the fluid passageways of the valve cage. In this arrangement, the valve is opened by lifting the plug off of the aforementioned seat ring, which thus allows the fluid to flow into the interior of the valve cage and thereafter radially outwardly through the fluid passageways of the valve cage. Conversely, the movement of the seating surface of the plug into sealed engagement with the complimentary seating surface of the seat ring facilitates a closed or shut-off condition for the valve.
Linear displacement control valves are often used to control flow and pressure in a process. However, such control valves possess certain deficiencies which detract from their overall utility. More particularly, in those applications where controllable low flow is a requirement, the valve trim is typically incapable of controlling flow accurately due, in large measure, to shortcomings in the existing designs of the plug or spindle integrated therein. In addition, in certain applications, the pressure drop in the control valve is severe enough to cause high velocity flow between the spindle and the inner diameter of the valve cage or disc stack. The high velocity flow eventually erodes the tip of the spindle, thus creating further problems in relation to low flow control using the valve. Thus, there exists a need in the art for a linear displacement control valve which is capable of providing low end flow control at a higher level of accuracy and/or wider rangeability. The present invention addresses this particular need, with the various features and advantages thereof being described in more detail below.